The invention disclosed and claimed herein is generally directed to a system for driving an MR gradient coil, so that the coil is operated in two or more different modes to produce pulses of selectively different gradient amplitudes and slew-rates during a single MR pulse sequence. More particularly, the invention pertains to a system wherein the gradient coil is operated in a given mode to produce gradient pulses having any gradient amplitude and any slew-rate up to a maximum gradient amplitude and maximum slew-rate, respectively, which correspond to the given mode. A preferred application of the gradient system of the invention is diffussion-weighted echo-planar imaging.
Persons of skill in the art have now recognized that an MR imaging sequence which combines diffusion-weighted (DW) imaging and echo-planar imaging (EPI) can serve as an effective clinical tool for the early diagnosis of acute stroke. As is well known, a DW imaging sequence is sensitive to particle motion resulting from diffusion, and comprises two successive gradient pulses of comparatively long time duration. Diffusion sensitivity is characterized by a parameter referred to as b-value which depends quadratically on DW gradient pulse amplitude. Because of the long time duration of the DW pulses, slew-rate, which is a measure of the increment of gradient amplitude in a unit time, is not of primary importance. However, it has been determined that use of a maximum gradient amplitude for the DW gradient waveform results in a shorter echo time (TE), for a given b-value, and therefore provides a higher signal-to-noise ratio (SNR). In one wholebody MR scanner, for example, TE can be reduced from about 99 msec to 72 msec when the amplitude of the DW gradient waveform is increased from 22 mT/m to 40 mT/m, for a b-value of 1100 sec/mm.sup.2. Such reduction in TE results in a considerable increase in SNR for brain tissue. Moreover, reduced TE decreases repetition time, and therefore increases volume coverage for the same imaging time. Alternatively, higher gradient amplitude allows a higher b-value with greater attendant diffusion sensitivity for a given TE. Using a higher gradient amplitude also makes DW trace imaging more feasible.
In EPI, a series of bipolar trapezoidal gradient pulses is used for data acquisition. It is very desirable to provide a high slew-rate, for successive pulses of the series, to reduce the spacing therebetween. In the readout of a combined DW-EPI sequence, a reduced spacing results in smaller off-resonance effects, such as image distortion and blurring. Moreover, it is anticipated that a substantial increase in slew-rate, over the currently used slew-rate of 120 T/m/sec, could provide these benefits in single shot EPI, which is the imaging method of choice for DW imaging.
From the above, it would appear that significant increases in both gradient amplitude and slew-rate would be very desirable in a combined DW-EPI sequence. However, simultaneously increasing both parameters could cause peripheral nerve stimulation and be in violation of FDA regulations. As is well known by those of skill in the art, the Reilly curve defines the limits of gradient amplitude and slew rate which are likely to result in nerve stimulation. If the values selected for the amplitude and slew-rate of a particular gradient pulse collectively exceed limits established by the Reilly curve, undesirable peripheral nerve stimulation could occur. Also, systems currently available in the prior art to provide two slew-rates generally require two different coils for each gradient axis. It would be very desirable, both for simplicity and to reduce the cost of gradient amplifiers, to provide a system which could operate a single gradient coil to produce pulses of different combinations of maximum gradient amplitudes and slew-rates.